A method of the kind mentioned above is disclosed for example in U.S. Pat. No. 3,962,866. In this method, the difference of the amplitudes of the signals of two probes is formed of which one probe is disposed forward of the catalyzer and the other probe is disposed rearward of the catalyzer. The difference formation is only undertaken as long as the control is active. In this case, the signal of the probe forward of the catalyzer oscillates with a certain amplitude while the signal of the probe rearward of the catalyzer exhibits a rather constant value as long as the catalyzer is new. However, for a catalyzer which has aged, the signal of the probe rearward of the catalyzer also oscillates and with increasing amplitude for increasing age. The above-mentioned measured difference therefore becomes ever smaller with increasing age. If this difference drops below a threshold value, then a warning signal is emitted which indicates that the catalyzer should be exchanged.
U.S. Pat. No. 4,622,809 discloses a further method wherein, with the aid of a probe forward of the catalyzer, the control is adjusted to the smallest possible amplitude by the probe. Then, the amplitude of the signal of a probe rearward of the catalyzer is measured. The interrelationships described above between this amplitude and the aging condition of the catalyzer apply identically. As soon as the amplitude of the signal of the probe rearward of the catalyzer exceeds a threshold value, an indication is provided that the catalyzer should be exchanged. However, a signal of this kind is obtained also when the amplitude remains below the above-mentioned threshold value but the mean value of this signal does not lie within a pregiven mean value window.
In addition to the methods described above, other methods are known which all are characterized in that for a lambda controlled internal combustion engine, the signals of the probes forward and rearward of the catalyzer are set into relationship with each other. This fact that these known methods have been further modified shows that the problem to reliably determine the aging condition of a catalyzer remained.